But Genesis tells us that the Spirit was in the world from the beginning of creation. The Spirit moved upon the face of the waters when God first made heaven and earth. From that beginning all throughout the Old Testament, the Holy Spirit is present in the world.*
The first thing to notice is that, in the Old Testament, the Holy Spirit is mentioned as coming only on a few particular people, often just people marked out for some particular purposes by God. But after Christ sends the Spirit at Pentecost, the Spirit comes on every single Christian. As the Second Reading explains, every person who says (and believes) that Jesus is Lord has the Holy Spirit dwelling in him.
Furthermore, after Pentecost, the Spirit that comes is the Spirit of the resurrected Christ, as well as the Spirit of the Father. And so when, after Pentecost, Christ’s followers are filled with the Spirit, they are unified with Christ, because his Spirit is within them.
At Pentecost, then, the Spirit comes on all those who believe in Christ; and it comes on them forever, for their whole lives, provided only that they remain in Christ. Those who have the Holy Spirit dwelling within them are made into the body of Christ, because the Spirit of the resurrected Christ is in them.
So that is why the Spirit comes at Pentecost to inaugurate the new age which God promised long ago. Through his prophet Joel (Joel 2:28), God promised that in the messianic age God would pour out his Spirit on all his people; and then God himself would be the hope and the strength of his people (Joel 3:16).We are included in that promise too. By God’s spirit that dwells in us, we are made one with our risen Lord.